Question

The pH gradient established by the electron transport chain in photosynthesis:

a.)  Exists between the matrix and the mitochondrial intermembrane space

b.)  Sets up a voltage gradient that drives GTP formation

c.)  Is established by electron transfer from FADH2 and NADPH from the TCA
cycle to oxygen

d.)  Occurs in part because Fe-S clusters can only transport electrons and not protons

e.)  None of the above 

Answer 1

1. Energy obtained through the transfer of electrons down the ETC(ELECTRON TRANSPORT CHAIN) is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (ΔpH) across the inner mitochondrial membrane (IMM).

This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential (ΔΨ_M). It allows ATP synthase to use the flow of H⁺ through the enzyme back into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate.

Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q (ubiquinone; labeled Q), which also receives electrons from complex II (succinate dehydrogenase; labeled II).

Q passes electrons to complex III (cytochrome bc₁ complex; labeled III), which passes them to cytochrome c (cyt c). Cyt c passes electrons to Complex IV (cytochrome c oxidase; labeled IV), which uses the electrons and hydrogen ions to reduce molecular oxygen to water.

Four membrane-bound complexes have been identified in mitochondria.

Each is an extremely complex transmembrane structure that is embedded in the inner membrane.

Three of them are proton pumps.

The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers.

Mitochondria are present in respiration of cells not in photosynthesis

2. Proton pumps are the heart of the electron transport process. They produce the transmembrane electrochemical gradient that enables ATP Synthase to synthesize ATP.

3. TCA cycle : (Tri Carboxylic Acid cycle or citric acid cycle)    ---- Respiration

Acetyl – CoA + 3 NAD⁺ + Q + GDP + P_i + 2 H₂O   -------------->

                                               CoA-SH + 3 NADH + 3 H⁺ + QH₂ + GTP + 2 CO₂

4. Fe-S clustes carry electrons

partly d is true but not the exact reason.

The exact reason is

When the excited electron is passed from one carrier to the other carrier, energy is released. This energy is utilised to pump protons from the the thylakoid membrane into the thylakoid space which creates the proton gradient.

The absorption of 4 photons by photosystem II generates 1 molecule of O₂ and releases 4 protons into the thylakoid lumen. The 2 molecules of plastoquinol are oxidized by the Q cycle of the cytochrome bf complex to release 8 protons into the lumen. Finally, the electrons from 4 molecules of reduced plastocyanin are driven to ferredoxin by the absorption of 4 additional photons. The 4 molecules of reduced ferredoxin generate 2 molecules of NADPH. Thus, the overall reaction is:

2H₂O + 2 NADP⁺ + 10H⁺_stroma    ------->        O₂ + 2 NADPH + 12 H⁺_lumen

The 12 protons released in the lumen can then flow through ATP synthase. Given the apparent stoichiometry of 12 subunit III components in CF₀, we expect that 12 protons must pass through CF₀ to complete one full rotation of CF₁ and, hence, generate and release 3 molecules of ATP. Given this ratio, the overall reaction is

2H₂O + 2 NADP⁺ + 10H⁺_stroma       ------>         O₂ + 2 NADPH + 12 H⁺_lumen

3ADP^3- + 3P_i^2- + 3H⁺ + 12H⁺_lumen   ------->    3 ATP^4- + 3H₂O + 12H ⁺_stroma

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2NADP⁺ + 3 ADP^3- + 3 P_i^2- + H⁺     -------->       O₂ + 2 NADPH + 3 ATP^4- + H₂O